Fabrication of advanced device structures, such as three dimensional (3D) semiconductor structures with complex surface topology, high packing density and materials with finely tuned chemical compositions is replete with complex technical challenges. Reactive ion beam etching (RIBE), chemically assisted ion beam etching (CAIBE) and other plasma dry etching methods use a combination of energetic ions like Ar+, Cl+ and CF3+ and reactive neutrals species like radicals Cl, F, and CF3 or excited state species like Cl* or F* to remove solid substrate material as volatile gas phase molecular species. In one example solid silicon, Si(s) may be converted into gas phase silicon tetrafluoride, SiF4(g) by reaction with a combination of fluorine ions, F+, and fluorine atoms, F:Si(s)+F++3F→SiF4(g)  (1)
Process conditions in known plasma apparatus may be selected so that ions extracted from a plasma generate a generally anisotropic etch where trajectories of the ions may follow electric fields that extend to a substrate. As a result the trajectories of ions under certain conditions may be relatively uniform, such as vertical trajectories, with respect to a (horizontal) substrate surface. Reactive radicals generated in a plasma exhibit a random distribution of trajectories and angles of incidence at the wafer surface and etch patterned surfaces isotopically. Isotropic etching of a patterned surface may result in undercutting of the pattern hard mask, scalloping or horizontal etching of feature sidewalls, degradation of sidewall angle and chemical attack causing composition change in reactive solids like oxides, metals, semiconductors or alloys or such materials. Isotropic etching by radicals in known reactive ion etching processes may be much slower (>10×) than anisotropic ion-driven etching, enabling adequate pattern transfer from a mask to substrate. Remaining challenges in such processing include the unwanted process-induced damage and composition change commonly observed. Another challenge is to control radical flux at the substrate surface in order to reduce isotropic etching and process induced damage by radicals.
With respect to these and other considerations the present improvements may be useful.